1. Field of the Invention
The present invention relates generally to maintenance of steam generators of nuclear reactor power plants and, more particularly, is concerned with an end effector apparatus for moving and positioning a tool for plugging a defective heat exchanger tube of the steam generator.
2. Description of the Prior Art
There are many situations in which a hazardous environment limits human access to various locations. One such situation occurs in the maintenance of operating steam generators of nuclear reactor power plants. A typical steam generator in a pressurized water nuclear reactor (PWR) includes a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the ends of the tube bundle opposite its U-like curvature, and a dividing plate that cooperates with the tube sheet to form a primary fluid inlet plenum at one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle.
The steam generators of the PWR receive both primary and secondary fluids to produce steam for subsequent production of electricity in a conventional manner. The primary fluid, after being heated by circulation through the nuclear reactor core, enters the steam generator through the primary fluid inlet plenum. From its inlet plenum, the primary fluid flows upwardly through the one end of the tube bundle supported by the tube sheet, through its U-like curvature, downwardly through its opposite other end also supported by the tube sheet, and into its outlet plenum. At the same time, a secondary fluid, known as feedwater, is circulated around the U-shaped tube bundle in heat transfer relationship therewith, thereby transferring heat from the primary fluid in the tubes of the bundle to the secondary fluid surrounding the tube bundle and causing a portion of the secondary fluid to be converted to steam. Since the primary fluid contains radioactive particles and is isolated from the secondary fluid by the U-shaped walls of the tubes and by the tube sheet, it is important that the tubes and the tube sheet be maintained defect-free so that no leaks will occur in the tubes or in the welds between the tubes and the tube sheet thus preventing contamination of the secondary fluid by the primary fluid.
Occasionally it is necessary to either inspect or repair the tubes of the bundle or tube sheet welds by way of access through the primary fluid inlet and outlet plena. For this purpose manways are provided in the vertical shell so that working personnel may enter the inlet and outlet plena to perform operations on the tubes and tube sheet. However, since the primary fluid, which is generally water, contains radioactive corrosion products, the inlet and outlet plena become radioactive which thereby limits the time that working personnel may be present therein. Accordingly, it would be advantageous to be able to perform operation on the tubes and tube sheet without requiring the entry of working personnel.
There are several mechanisms known in the art that attempt to provide a solution to this problem. Some of such mechanisms are described in U.S. patents to Dent et al (U.S. Pat. No. 4,303,368), Rieben et al (U.S. Pat. No. 4,369,662) and Kucherer et al (U.S. Pat. No. 4,390,042), which are as to the assignee of the present invention, and in patents and applications memtioned in these patents. While these mechanisms appear to operate satisfactorily under the limited range of conditions for which they were designed, a need still exists for an improved mechanism which reduces personnel radiation exposure, shortens plug installation time, offers greater versatility and speed, and requires less skill and training for the operators of the equipment.